Lake margin sedimentary systems can provide highly sensitive records of sedimentary response to climate change. The Middle Old Red Sandstone of Northern Scotland comprises a thick succession of cyclic lacustrine sediments. Within this succession the deepest lake phase, the Achanarras fish bed, allows bed-scale correlation over 160 km across the basin. This provides a unique opportunity to examine the character of synchronous lake margin deposits, and their response to climatically driven lake level fluctuations, across a large continental basin. Detailed characterisation of two separate lake margin systems was carried out utilising multiple sections in western Orkney, in the north, and Easter Ross, in the south. Seven facies have been recognised, which include upper and lower shoreface, deep lake, shallow lake, playa, turbidite and fluvial facies. Differences in vertical and lateral facies stacking patterns reflect the response of these systems to climatically driven fluctuations in lake level. Comparison of the northern and southern systems examined highlights the variable response of lake margin systems to the same climatic change and related lake level fluctuations. In the south, a greater fluvial influence is recognised on the development of the lake margin successions, whereas in the northern example, which lay on the downwind margin of the lake, shore zone facies are more commonly developed. The variability recognised can be accounted for by regional variations in sediment supply, coastal physiography, lake size, bathymetry and potential fetch. Lake level stability is also recognised as a major control on the development of lake margin sedimentary systems, as is the linked or unlinked relationship of the catchment and the lake basin climate for which a conceptual model is proposed.
M. J. Newman is gratefully acknowledged for his identification of fossil fish specimens from the Easter Ross coast which helped in confirmation of the position of the Achanarras fish bed in this region. The Inverness Field Club and the Carnegie Trust are thanked by SDA for financial support of fieldwork undertaken. Nigel Trewin is acknowledged for inspiring a long-lived interest in the Devonian of Northern Scotland which has led to some fantastic journeys. Gary Nichols and Alberto Saez are thanked for constructive reviews
- climatic forcing
- lacustrine cycles
- lake margin
- Orcadian Basin